I set up OpenVPN server on AC68U 384.8_2 successfully and could connect to it using Win 10 OpenVPN client.
Default setting: "LAN only" - but I found that I could surf on the web and use my LAN folders too... I thought this setting was for restricting web surfing - this is what I really needed, but it did not work.

I started experiencing and set this setting to "Internet Only" and applied setting. Now I can connect to the server, can surf the web and access LAN resources - BUT I CANNOT access the router GUI anymore...
I tried through the LAN IP, http:// https://, asuscomm.com ddns but I can't back to GUI.
However I can SSH into the router, list commands, but I don't know how to roll back to "LAN only" setting...

Thanks for your quick reply. Actually it is very useful for my other issues, good to know. After reading the post, I went back to file explorer, clicked on network, right clicked on AC68U router icon, selected Properties, then on the Network device tab clicked on Device webpage... and voilá: the GUI appeared!
Maybe the time solved the problem while I was reading the post

However on the other browser tabs it still wouldn't work, so it may have been a browser cache or network cache problem... i don't know.
So now it is SOLVED!